I. Field of the Invention
The present invention relates generally to radio astronomy, and the microwave antenna arrays and systems utilized in such installations. More particularly, the present invention relates to multi-element antenna arrays primarily adapted for astrophysical research and the Search for Extra-Terrestrial Intelligence (SETI), and methods for electrically aiming them.
II. Description of the Prior Art
It has long been recognized by those skilled in the art that multiple antennas can be combined together for increased receiver performance. The advantages are numerous and well-known. Various forms of prior art technology exist for combining the antennas.
For example, in astrophysical research and the electromagnetic Search for Extra-Terrestrial Intelligence (SETI), it has been the common practice to combine multiple dish antennas into an array, optimized to produce a specific beam geometry. Beam geometries tend to be highly application-specific. For example, drift-scan SETI receiving stations are best served by an antenna pattern that is somewhat broader in the declination axis than it is in right ascension. Such a beam pattern was implemented by the late Ohio State University xe2x80x9cBig Earxe2x80x9d radio telescope, circa 1964-1997, which was one of the great pioneers in SETI. Total power studies of the galactic core favor an opposite antenna pattern (that is, a geometry which is broader in right ascension than it is in declination). Targeted searches of individual stars and quasi-stellar objects require a spot beam, narrow in both planes.
At this time, in order to change from one beam pattern to another, different physical array geometries are used. Obviously this approach has limitations, especially when funds are limited, and thus only a single array geometry may be practicable. Alternatively, antennas may be physically relocated. This approach is obviously difficult, and often impractical. For example, the twenty-seven dish antennas at the multi-million dollar Very Large Array (VLA) in Socorro N. Mex. each weigh two hundred and thirty tons. To change this array between operating configurations, each of its dishes is moved along approximately thirty miles of railroad track. However, multiple diverse beam geometries often tend to be mutually exclusive. An adaptive antenna array, one that can operate in multiple geometric modes simultaneously, would be highly advantageous.
The advantages gained by combining multiple antennas into an array are well known, and fall into two broad categories: (a) improving sensitivity, and (b) improving resolution. The two most common ways of connecting multiple antennas into an array are (a) as a radiometer, and (b) into correlation detectors. (Burke and Graham-Smith, 1997). In the case of the radiometer connection, a single detector is connected to all of the antennas in the array via a branched feedline, which maximizes sensitivity by producing a single beam. The best known (though never implemented) example of this configuration is Project Cyclops (Oliver et. al., 1973).
With interferometers (Ryle, 1952), resolution is improved by combining the signals of two antennas which are separated by a specified distance (called the baseline). With dish antennas, the resulting gain is simply that which would be achieved by a single dish with a surface area equal to the sum of that of the two antennas. However, the angular resolution of such an interferometer is equivalent to that of a single dish with a diameter equal to the baseline. Thus, interferometers provide a modest improvement in sensitivity with a much greater increase in resolution. A multiple-antenna interferometer array may be constructed using a technique known as aperture synthesis. Each possible pairing of antennas in the array is accomplished by applying the outputs of the antennas to a multitude of correlator circuits. The correlator outputs may be combined to produce multiple beams, making it possible to image distant astrophysical objects with high levels of detail. Well-known multiple-antenna interferometers include the Very Large Array (Napier et. al., 1983) and the Giant Meter-Wave Radio Telescope (Swarup et. al., 1991). Both of these arrays arrange their antennas (27 in the case of the VLA; 30 at the GMRT) in a xe2x80x9cYxe2x80x9d configuration with extremely wide baselines, and use digital correlators to combine the signals from the multiple dishes.
The Mills Cross arrangement (Mills, 1963) consists of two line-type antennas, one oriented North-South and the other East-West. The former antenna produces a beam pattern which is narrow in declination and broad in right ascension. The latter produces a beam pattern which is broad in declination and narrow in right ascension. When signals from the two antennas are combined, a beam is produced which is narrow in both axes.
Bracewell and Swarup (1961) produced an array of thirty-two small parabolic dish antennas, oriented in a Mills Cross arrangement, to produce a pencil-beam interferometer with micro-steradian resolution. All of the antenna arrays described above achieve stated design goals of high sensitivity or high angular resolution. In each case, one and only one of these design objectives can be achieved, often at the expense of the other.
Known prior art concepts are discussed in the following references:
Bracewell, R. N., and G. Swarup, The Stanford Microwave Spectroheliograph Antenna: A Pencil Beam Interferometer, IRE Trans. Antennas and Propagation, vol. AP-9, pp. 22-30, January 1961.
Mills, B. Y., Cross-type Radio Telescopes, Proc. IRE Australia, vol. 24, pp. 132-140, 1963.
Ryle, M., A New Radio Interferometer and Its Application to the Observation of Weak Radio Stars, Proc. Royal Soc. London Ser. A, vol. 211, pp. 351-375, 1952.
Burke, B. F., and F. Graham-Smith, An Introduction to Radio Astronomy, Cambridge University Press, 1997.
Swarup, G., S. Ananthakrishnan, V. K. Kapahi, A. P. Rao, C. R. Subrahmanya, and V. K. Kulkarni, The Giant Meter-wave Radio Telescope, Current Science, vol. 60 no. 2 pp. 95-105, Jan. 25, 1961.
Napier, P. J., A. R. Thompson and R. D. Eckers, The Very Large Array, Design and Performance of a Modern Synthesis Radio Telescope, Proc. IEEE, vol. 71 no. 11 pp. 1295-1320, November 1983.
Oliver, B. M., and J. Billingham, eds., Project Cyclops, A Design Study of a System for Detecting Extraterrestrial Intelligent Life, NASA CR 114445, 1973.
The invention presents an array of small, dish antennas all united to accomplish specific beam patterning. Preferably the array comprises n individual antennas. Four subarrays, each with (n/4) individual antennas, are established in a cross-like formation, with a subarray running north, south, west and east. The array resembles the Bracewell and Swarup array in physical configuration. Unique circuitry is added to allow it to operate both as a total-power radiometer, and as a correlated interferometer, simultaneously. These multiple operating modes allow the array to achieve both high sensitivity and high angular resolution, fulfilling a variety of research objectives.
A solution is provided for electronically changing a complex, multiple-antenna array into different configurations yielding different beam patterns. In other words, radio signals derived from the four subarrays can be electronically processed and combined into a variety of beam patterns. These multiple patterns are synthesized through a combination of analog in-phase combining means, analog phase-quadrature signal combining means, and digital conversion and software correlation means. As different individual antennas forming each subarray all monitor the same broad portions of sky, such analog and digital processing of signals derived from the individual antennas can be processed not only to yield the composite observed target sought by the radio telescope, but can produce high angular resolution beam patterns subtending selected portions of the overall sky coverage.
Thus a basic object is to provide an adaptive antenna array system which can operate in multiple geometric modes simultaneously.
Fundamentally, it is desired to be able to electronically convert a radiotelescope and switch it between beam patterns.
Another fundamental object is to electronically aim an antenna system comprising multiple subarrays of multiple antennas.
A related object is to provide an adaptive antenna array of the character described that can achieve a beam pattern that is broader in the elevation axis than in the azimuth axis.
Conversely, another object is to provide an adaptive antenna array of the character described that can achieve a beam pattern that is broader in azimuth than in elevation.
A related object is to provide an electronic means of creating a beam geometry narrower in both azimuth and elevation that those of the individual antennas, so as to improve angular resolution, to aid in the study of individual stars, quasi-stellar objects, and other deep space targets.
Another object is to provide an array that can produce multiple, simultaneous spot beams, allowing detailed sky maps to be developed.
A major object is to simplify the changing from one beam pattern to another.
A still further object is to maximize sky coverage while minimize the necessity of physically moving antennas.
Another basic object is to provide a highly versatile multiple antenna array system suitable for use by universities or layman, public and private owners, and/or professional and amateur observers.
Recognizing that some beam patterns in microwave antenna arrays are best achieved by digital processing, and that different beam patterns are better achieved through analog processing, a final object is to provide multiple signal outputs from each antenna in the array, to allow simultaneous analog and digital processing of the available signals.